To determine which viral genes control vaginal transmission of HIV, defined viral constructs must be tested in an animal model. The SIV/rhesus macaque system was used to test the role of the HIV envelope gene in vaginal transmission. SHIVS containing the envelope glycoproteins of two HIV-1 strains, HXB2 and 89.6, In an SIVMAC239 backbone have been generated. The HXB2 virus is the prototype of the T-cell tropic viruses. The 89.6 Virus is a highly cytopathic variant of the macrophage-tropic viruses. These chimeric viruses were made in Dr. Sodroski's laboratory and the infectious stocks were produced in Dr. Lu's laboratory. Both SHIV (HXB2) and SHIV (89.6) initiate intravenous infection of rhesus macaques. At the CRPRC, two mature female rhesus macaques were inoculated intravaginally 4 times (twice a week for 2 weeks) with the HXB2 SHIV clone (2500 TCID50). Both animals remained virus isolation negative and seronegative for the duration of the 6 month observation period. Subsequently these 2 animals and 2 naive adult female rhesus macaques were inoculated intravaginally 3 times (once every 3 days) with the 89.6 SHIV clone (1800 tcid50). Both of the naive animals and 1 of the HXB2 exposed animals were viremic by 14 days pi. These 3 animals seroconverted to SIV gag/pol and HIV env antigens at 8-10 weeks pi. The other HXB2 exposed animal remains virus isolation negative and seronegative during 4 months of observation. These results demonstrate vaginal transmission of SHIVS for the first time and, although based on small numbers of animals, suggest that SHIV (89.6) Possesses properties that allow for more efficient infection via the vaginal route compared with SHIV (HXBC2). Thus, the viral determinants required vaginal transmission can be defined using these chimeric viruses.